1. Field of the Invention
The present invention relates generally to a system for maintaining pressure in a pressurized storage vessel as liquified gases are withdrawn therefrom. More particularly, the present invention relates to a vaporizer for reliably producing sufficient carbon dioxide vapors from liquified carbon dioxide to replace exiting liquids in the pressurized storage vessel.
2. Known Art
As will be appreciated by those skilled in the art, carbon dioxide is typically liquified for convenient transport between production facilities and users. The delivered liquid carbon dioxide is normally stored at an elevated pressure in a suitable pressure vessel to maintain its liquidity.
The stored liquid carbon dioxide is used in a wide variety of commercial and industrial processes. For example, "blast freezing" with liquid carbon dioxide is employed by many food processing facilities to quickly freeze food products and the like. Often the carbon dioxide liquid must be withdrawn in large quantities from the storage vessel for use in the process (especially blast freezing and the like). However, the withdrawal of liquids from the vessel causes a pressure drop in the vessel. The resultant pressure drop causes the inner temperature of the storage vessel to rise, which results in further vaporization of the valuable liquified carbon dioxide. This vaporization occurs every time liquid is withdrawn and depletes the quantity of liquid available for the facility. As such, this vaporization process is undesirable.
One method of preventing undesirable carbon dioxide vaporization is to maintain a constant pressure in the storage vessel. This is accomplished by simultaneously adding replacement vapor to the vessel to replace the exiting liquid. Thus, as liquids are withdrawn, carbon dioxide vapors are added so that the interior pressure remains constant in the vessel. A popular way of obtaining carbon dioxide vapor to replace the exiting liquid is to vaporize a small portion of the exiting liquid. Conventional vaporizers have heretofore been employed to accomplish this task. Examples of such devices are shown in several patents of general relevance.
U.S. Pat. No. 2,343,727 to Zenner discloses a vaporizing device for vaporizing volatile liquids such as oxygen, nitrogen and the like. The vaporizer uses a unitary manifold disposed inside a shell. U.S. Pat. No. 4,590,770 to Howard discloses a cryogenic liquid heat exchanger for vaporizing cryogenic liquids. This device employs a single tube for vaporizing the liquid using indirect heating.
U.S. Pat. No. 3,712,073 shows another interesting vaporizer as well. This vaporizer is associated with storage tanks and shows the associated pumping and piping with such a conventional arrangement. U.S. Pat. No. 5,243,821 shows another process of general relevance. The process produces gases over a wide range of flow rates.
These devices may work well for their intended uses with small installations. However, no known device works well for large installations, particularly those large installations processing food. For example, large poultry processing facilities use liquified carbon dioxide to maintain and/or quickly freeze processed poultry. Such plants typically require 30-60 tons of liquid carbon dioxide daily. Furthermore, given the strict FDA standards for the time periods and temperatures during which poultry must be processed and packaged, it is critical that the liquified carbon dioxide supply system function adequately at all times. For example, current FDA guidelines require that the chicken carcasses be maintained at 34.degree. fahrenheit and that they be packaged promptly after blast freezing.
Another problem associated with the known art is its unreliability. The known vaporizers often suffer from "freeze-ups" where they cease working properly. During such failures, the entire processing facility must often cease all work until the liquid carbon dioxide supply can be reestablished. As may well be imagined, such work stoppages cripple production and should be avoided.
However, the known art fails to provide a suitable vaporizer that efficiently vaporizes the liquid carbon dioxide in sufficient quantities to meet the daily demand of large commercial installations. An improved vaporizer that facilitated maintenance and repair would also be desirable. An ideal vaporizer would resist freeze-ups to ensure a constant supply of liquid carbon dioxide. It is also desirable to provide an improved vaporizer that reliably produces significant quantities of carbon dioxide vapor without requiring significantly higher steam and/or electrical energy and is of minimal size.
Thus, a need exists for an improved vaporizer to overcome the perceived deficiencies.